How about a simple differential amplifier without any transformers like this.

You can connect this as a SE or balanced input and output as you wish.
The low frequency response can be lowered to 20 Hz by replacing C2 and C3 with 220 nF if necessary.

You'd need some complex switching arrangement to provide both balanced and unbalanced inputs to that. It also has high output impedance. This isn't a design thread as far as I know. He just wants to something easy and mostly ready made I think.

Well, I sort of see the "not DIY enough" point, and I'd certainly be capable of soldering "through hole" components to a PCB and wiring the PCBs, switches and other parts together in a case myself, but int that kit, the boards happen to be populated. The reason I mentioned these kits is that they seem like an easy way to get everything I need. Given that the components are decent, of course.

...And I'd probably get a better looking amp that way, FWIW.

What I'm afraid of is that I might get an ugly, noisy and expensive amp if I try to do it the "proper" way. Especially when I want both balanced and several single ended inputs on it.

I found an image showing my almost finished (for several years now) guitar amp as a reference. Not really that pretty:

It sounds like you'd be better off with a complete kit even if it's not ideal or perfect or totally awesome. Like I said, you can adapt an existing single ended kit to receive balanced inputs and to provide balanced outputs with a few transformers. Otherwise, you'd probably have to learn a lot more about circuit design.

Also, if a fully electronically balanced design is used, then you need much more complicated input switching and volume controls as well. My preference is to switch any balanced signal to single ended inside the box and then convert it back to balanced for the output for just these reasons.

No. It is low for this purpose.
For feeding 50 kohms load, 2.1 kohms output impedance is more than sufficiently low.

I don't remember if he stated the input impedance for his amplifiers. I thought they were solid state, which usually means an input impedance of 10kohms. He may not even know. At any rate, 2.1kohms is high for an output impedance.

I don't remember if he stated the input impedance for his amplifiers. I thought they were solid state, which usually means an input impedance of 10kohms. He may not even know. At any rate, 2.1kohms is high for an output impedance.

I actually did.

Quote:

I'd like to try a tube preamplifier with my two mono bridged Abrahamsen V4.0UP power amplifiers. In bridged mode they have a 50KOhm input impedance balanced. (100KOhm with SE adapters) They also each have a 1000VA toroidial transformer and a 100.000uF capacitor bank. Bridged they should give about 240W each @8Ohms, I guess. I don't know if this matters, but anyway...

This is straight from the designer, Per Abrahamsen, and I trust it's accurate.

There's no need to argue, guys. I appreciate all the help from all points of view. Both kinds of project are tempting, but right now, I'm leaning towards the instant gratification of a kit, or at least something based on circuit boards.

That being said; building something like described on artosalo's schematic sounds very cool, and I would like to try that as well, but maybe as my next project. Sometime after I get my gitar amp working, for instance.

I'm all for learning, of course, but a sucker for instant gratification as well. ;D